A conventional four-phase hybrid type pulse motor is shown in FIGS. 1 and 2. A rotary shaft 11 has affixed thereto a permanent magnet 12 and rotor cores 13 coupled to both ends of the magnet 12. The rotor cores 13 have teeth 14 formed around their peripheral surfaces at the same angular pitch and at the same angular positions. The rotary shaft 11, the permanent magnet 12 and the rotor cores 13 constitute a rotor 15.
Disposed about both rotor cores 13 is a cylindrical stator yoke 16, which has eight magnetic poles 17 projecting out from its interior surface at equiangular intervals in opposing relation to the peripheral surfaces of the rotor cores 13. Each magnetic pole 17 facing the rotor 15 has teeth 18 formed in its projecting end face at the same angular pitch as that of the teeth 14, and each magnetic pole 17 has wound thereon a coil 19. The stator yoke 16, the magnetic poles 17 and the coils 19 form a stator 21. The rotor 15 and the stator 21 are disposed in a housing 22, with the rotary shaft 11 rotatably supported relative to the housing 22 by means of bearings 23.
The operation of this conventional pulse motor is well-known, and hence will be described in brief.
FIG. 3 is a development elevation of the pulse motor depicted in FIGS. 1 and 2, showing mainly the south pole side of the permanent magnet 12. Greek numerals I, II, III and IV indicate the phase numbers. The teeth 18 of adjacent magnetic poles are displaced one-fourth the pitch apart. In the state depicted in FIG. 3, when current flows in the coils 19 of the magnetic poles 17 of the phase numbers I and III in the direction shown, the teeth 18 will be magnetized with polarities as shown. This causes the magnetic flux emanating from the permanent magnet 12 and the flux from the coils 19 to flow between the teeth 14 of the rotor core 13 and the teeth 18 of the stator 21 as indicated by the solid lines and the broken lines, respectively. As a result, the magnetic fluxes cancel each other in the teeth 18 of the phase number III, and hence no attractive force will be generated, while an attractive force will be produced in the teeth 18 of the phase number I in which the magnetic fluxes are added together, causing the rotor 15 to move to the right in FIG. 3 by a distance equal to a quarter of the pitch. Then, the teeth 18 with coils 19 of the phase numbers II and IV, in which no current flowed, come to bear the same positional relationships to the teeth 14 of the rotor 15 as did the teeth 18 of the phase numbers I and III before the movement of the rotor; by supplying current to the coils 19 of the phase numbers II and IV, the rotor can similarly be moved. By repeating this operation an attractive force is always produced between the teeth 18 of any one of the phase numbers and the teeth 14 of the rotor 15, causing the rotor 15 to continue rotating in one direction by steps of a distance equal to one-fourth of the pitch.
With the conventional pulse motor structure described above, the reduction of its diameter is difficult because it calls for the reduction of the diameter of the rotor 15 and/or the diameter of the stator 21, which incurs a decrease in the torque of the motor. On the other hand, reduction in the size of the motor in the axial direction of the rotary shaft 11, that is, the thinning of the motor, is relatively easy and the decrease in the torque counterbalances to the miniaturization, so that a relatively large number of such motors are now on the market. With this type of motor structure, however, since the rotary shaft 11 needs to have a certain length for mounting thereon the rotor cores and so on, miniaturization of the conventional pulse motor in which the length of the rotary shaft 11 is equal to the length of the motor in its thickwise direction (in the direction of its diameter) cannot be readily effected.
An object of the present invention is to provide a pulse motor of a shape and dimensions unachievable in the past, through reduction of its size in a direction perpendicular to the rotary shaft.